The desirability of combining the optimum characteristics of oriented thermoplastic materials, such as maintenance of crease and tear resistance, with the dyeability and sorptivity of natural materials has been recognized for some time. Attempts in this direction have usually resulted in blending the two materials together so that there is an averaging of the properties of the materials rather than an optimization of each of the component's most desirable properties.
Coextrusion of two different materials to form a side-by-side bicomponent fiber has been done extensively, primarily to develop a crimped product. For example, U.S. Pat. No. 2,439,813, Kulp, discloses such a product, where both components are viscose of different contractivity due to different aging times and different concentrations of cellulose, carbon disulfide, or sodium hydroxide. Sheath core structures have also been formed, again usually for crimping purposes. For example, U.S. Pat. No. 3,458,615, Bragaw, discloses a coextrusion of two streams in the molten state and any orientation to be developed will be induced downstream from the die. The Bragaw patent is directed to the production of light guides where a well controlled smooth interface is critical to maintaining internal reflection of the light passing through the core and reflected off the surface. U.S. Pat. No. 2,932,079, Dietzsch, discloses a sheath core structure which must contain at least two cores of different materials and a sheath of a third material. This is so that crimp may be developed by differential thermal contraction of nonconcentric core and sheath layers. U.S. Pat. No. 2,989,798, Bannerman, is also involved in the production of a sheath core fiber in which both layers are polyamides. The core polyamide is chosen or modified to be more dye receptive. U.S. Pat. No. 2,063,180, Meyer, involves a coextrusion process in which an inner stream consisting of a volatile solvent carrying a coloring substance passes through a wick and is subsequently covered by a viscose solution. During spinning the volatile solvent diffuses through the forming rayon leaving behind only the coloring substance. The inner core would not exist as a discrete region since the dye would form a gradient into the rayon.
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